oniro-linux: Add sftp-server package

Modern ssh clients use SFTP protocol and refuse to connect using
SCP protocol (dropbear uses) by default. Adding sftp-server to allow
sftp connections in Oniro.
This requires workaround for [Yocto #14858] to be applied for every
Oniro images, not only extra-tests.

Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>